This invention relates to a concentration process wherein normal cis olefins are concentrated in a hydrocarbon composition.
It is well-known that normal olefins have desirable properties over branched olefins as excellent starting materials for linear plasticizer alchols. These alcohols exhibit better low temperature properties than non-linear plasticizer alcohols and also increased bio-degradability. Separation of normal olefins from branched olefins is accordingly exceedingly desirable.
It is well-known also that certain natural and synthetic zeolites have the property of preferentially absorbing certain types of hydrocarbons. These zeolites, known as molecular sieves, have crystalline structures containing a large number of pores of uniform size. In different zeolites, these pores can vary from 4 Angstroms (A.degree.) to 15 A.degree. or more in diameter but in any one zeolite the pores will be of substantially uniform size. Specific zeolites which are referred to as molecular sieves accordingly can be utilized for separating specific hydrocarbons on the basis of the size and shape of the adsorbate molecule. These molecular sieves have a sorption area available on the inside of the uniformly sized pores. With such an arrangement molecules of a certain size and shape enter the pores and are adsorbed while larger or differently shaped molecules are excluded.
It is known that molecular sieve type 5A is capable of separating straight chain hydrocarbons from non-straight chain hydrocarbons by the acceptance-exclusion principle based on molecular size. Many processes based upon this have been successfully applied for separations of paraffinic hydrocarbon mixtures. However, because the acceptance-exclusion principle operates based on molecular size, the efficiency of separating branched olefins from normal olefins has suffered because of the difficulty of separating the cis isomer configuration from branched olefins.
The cis isomer of normal olefins in molecular configuration is more akin to branched olefins than to the trans isomer of normal olefins. Accordingly, separation of the cis isomer from branched olefins using molecular sieve techniques in the prior art has not been as complete as has been the separation of the trans isomers from branched olefins.
The molecular sieve effect whereby cis isomers are adsorbed less readily by synthetic zeolites of the CaA type has been reported and is generally known, (Petryaeva et al., Izu. Akad. Nauk SSSR, Ser. Khim. 1967(8), 1860-2, CA 6640j (1968)), although the NaX zeolites indicate an adsorption of all hexenes equally well, linear and non-linear.
U.S. Pat. No. 2,866,835 to Kimberlin et al. relates to a process for separating olefins from hydrocarbon streams using molecular sieves with pore diameters of about 4.5 to 5.5 Angstrom (A.degree.) units. Kimberlin taught that cis olefins in the original stream were not significantly adsorbed by the sieve and that the sieve adsorbate olefins were substantially completely of the trans type. The cis olefins were not preferentially separated from the hydrocarbon stream by being adsorbed by the molecular sieve.
U.S. Pat. No. 2,988,578 to Fleck et al. relates to a process for the isomerization of hydrocarbons and the separation of the cis-trans isomers by use of a 5 A molecular sieve. The mixed isomerized olefins are preferentially adsorbed to result in a substantially higher trans content over the cis content rather than a racemic mixture.
U.S. Pat. No. 2,850,549 discloses the separation of cis and trans olefin isomers wherein the separation is carried out over a long period of time, e.g., 24 hours.
U.S. Pat. No. 3,524,895 discloses a method for the separation of cis and trans hydrocarbon isomers which is achieved by passing a mixture thereof through a crystalline aluminosilicate adsorption zone for a relatively short adsorption period. It has been found that cis isomers have a substantially lower adsorption rate than do trans isomers and that therefore trans isomers can be separated thereby.
The lesser adsorption of the cis isomer by synthetic zeolites results in a loss of substantial quantities of the cis isomer in that cis isomer is not separated from the hydrocarbon stream as well as is the trans isomer. A method has been found whereby the cis isomer can be recovered in good yield from branched olefins. Overall concentration of cis and trans isomers is improved over the concentration obtained of cis-trans isomers from the hydrocarbon stream without use of the method of the instant invention.